1. Field of the Invention
This invention relates to object distance detecting apparatus and more particularly to an active type object distance detecting apparatus whereby pulse light such as infrared rays is projected onto an object to be photographed and the object distance is detected on the basis of the reflected light from the object.
2. Related Background Art
Object distance detecting apparatus used for autofocus (abbreviated as AF hereinafter) apparatus of still cameras and video cameras are largely divided into two systems. One of the systems is a passive system utilizing the luminance distribution information of the object and the other is a so-called active system having a self light projecting means and measuring the distance by the reflected light of the projected light signal. The active system is simple in the formation, is low in cost and therefore is high in the prevalence rate but has the greatest defect that the farther the object distance, the smaller the size of the reflected light, the AF operation will become inaccurate due to the deterioration of the S/N ratio and therefore the measurable range will be limited to a comparatively near distance. Particularly, the AF operation will be made by only the noise component within the circuit for such object said to be infinitely far as a scenery in which no reflected light returns at all but the noise will occur like random numbers and therefore the larger the distance, the higher the possibility of causing a mis-measurement of the distance.
Therefore, there is already suggested a means whereby, though likely to be influenced by the reflection factor of the object to be photographed, the light amount, that is, the intensity of the total reflection light entering a light receiving means is made a signal and is compared with a reference voltage to judge the infinitely far distance (See the publications of Japanese Patent Applications Laid Open Nos. 228212/1984 and 244807/1985). According to this means, it will be possible to judge distances up to a comparatively far distance.
However, in this prior distance measuring means, there has been a problem that, when the distance judged by the intensity of the reflected light of the object is to be minutely judged as divided into several steps, as many reference voltages and comparators as there are steps will have to be provided and the complication of the circuits and the increase of the cost will be inevitable.
The formation of an active type AF apparatus based on a generally known triangular distance measuring method shall be explained in the following with reference to FIG. 17. A light emitted by an infrared ray emitting diode IRED 1 will be condensed by a light projecting lens 2 and will be radiated toward an object 3 to be imaged and its reflected light will be made by a light receiving lens 4 to form an image on a well known position sensitive device (abbreviated as PSD hereinafter) 5. As a result, in this PSD 5, light currents I.sub.1 and I.sub.2 will be generated in response to the entering position and will be fed to an IC 6 for AF which will pulse-drive the above-mentioned IRED 1 through an IRED driving transistor 1A and will feed a CPU 7 with distance measuring data based on the light currents I.sub.1 and I.sub.2 from the above-mentioned PSD 5.
On the other hand, a light receiving device 8 for controlling the exposure (abbreviated as EE hereinafter) converting the brightness of the, object to an electric signal is combined with an IC 9 for EE to control a proper exposure. The above mentioned CPU 7 controls the sequence of the entire camera and operates the opening time of the shutter and driving a focus adjusting lens. The output of the CPU 7 drives a motor 11 through driver 10. Motor 11 serves as a power source to operate the shutter, wind up the film and pay out the lens.
The operating principle of the infrared ray projecting type triangular distance measuring method for measuring the object distance by the above-mentioned PSD 5 shall now be described herein. When the reflected light entering position is represented by x, the distance, that is, the base line length between the main points of the light projecting lens 2 and light receiving lens 4 is represented by s and the focal distance of the light receiving lens 4 is represented by f.sub.0, the object distance l will be given by EQU l=s.multidot.f.sub.0 / x (1)
The light currents I.sub.1 and I.sub.2 generated in the PSD 5 by the reflected light of the object by the IRED 1 are proportional to the reflected light intensity but the light current I.sub.1 /I.sub.2 is determined by only the entering light position x without depending on the reflected light intensity. If the total length of the PSD 5 is represented by t, EQU I.sub.1 /I.sub.2 =(t/2+x) / (t/2-x)
will be made. If the formula (1) is substituted in the above formula, EQU I.sub.1 / I.sub.2 =(t+2 s.multidot.f.sub.0 /l)/(t-2 s.multidot.f.sub.0 /l)(2)
will be made. Therefore, if the light current I.sub.1 / I.sub.2 of the PSD 5 is determined, the object distance will be readily determined.
If the above mentioned formula (2) is modified, ##EQU1## will be made and, in a near distance in which the light currents I.sub.1 and I.sub.2 are large enough, the information of the distance will be able to be determined at a high precision from this formula (3). That is to say, ##EQU2##
However, in a far distance, the light currents I.sub.1 and I.sub.2 of the PSD 5 will become smaller, the noise components I.sub.N1 and I.sub.N2 will relatively increase and therefore the above-mentioned formula (4) will have to be rewritten to be as in the following formula (5): ##EQU3##
When the above-mentioned formula (4) is made a theoretical value, the error .DELTA.1/l of 1/l determined by this formula (5) will be the formula (5)-formula (4), that is, ##EQU4## will be made.
Here, in the case of the above mentioned formula (6), as the ratio operation has been already made, however the number of times of the integration may be it is difficult to red increased, the error to zero.
On the other hand, though different from the triangular distance measurement, the distance may be measured by using the sum of the signal light currents I.sub.1 and I.sub.2 of the PSD 5.
Therefore, by Japanese Patent Application Laid Open No. 291111/1989, the present applicant has suggested an active type AF circuit wherein the distance is measured for a near distance object by using the operation of the ratio given by the above-mentioned formula (4) and for a far distance object by the result of the integration of the light amount.
The object distance detecting apparatus in the AF circuit of the Japanese Patent Application Laid Open No. 291111/1989 is formed as shown in FIG. 18 wherein the output result of the ratio I.sub.1 /(I.sub.1 +I.sub.2) operating circuit 15 and the output result of the adding circuit 16 operating the light amount integration are selectively processed in the same integrating circuit 20.
That is to say, the light currents I.sub.1 and I.sub.2 from the PSD 5 are input respectively into the amplifiers 12 and 13 of low input impedances, have the DC current component based on the constant light separated and are then amplified. The respective outputs are input into the operating circuit 15 and adding circuit 16. The adding circuit 16 adds the light currents I.sub.1 and I.sub.2 and the output I.sub.1 +I.sub.2 is fed to one input terminal of the comparator 18. A comparing signal I.sub.ref output from a comparing signal generating circuit 17 is fed to the other input terminal of the comparator 18.
The operating circuit 15 operates I.sub.1 /(I.sub.1 +I.sub.2) corresponding to the ratio of the light currents I.sub.1 and I.sub.2 and inputs its output, into the integrating circuit 20 through the switch 19. The output of the above-mentioned adding circuit 16 and the comparing signal I.sub.ref are fed also to the subtracting circuit 23 which generates a signal I.sub.ref -(I.sub.1 +I.sub.2) corresponding to the intensity of the reflected light for the measurement of a far distance. The output of the subtracting circuit 23 is input into the integrating circuit 20 through a switch 22. The output of the integrating circuit 20 is taken out as AF data. By the way, at the time of starting an integration, a predetermined integration starting voltage will be given to the integrating circuit 20 by the resetting circuit 21.
The above-mentioned switch 19 is controlled by the output of the comparator 18 and the switch 22 is controlled by the inverted output of the comparator 18. The integrating circuit, 20 operates an integration based on either one of the output of the operating circuit 15 and the output of the subtracting circuit 23 in response to the size relation between I.sub.1 +I.sub.2 and I.sub.ref.
Here, in case I.sub.1 +I.sub.2 is larger than I.sub.ref, the object will be at a near distance and the S/N ratio of the light currents I.sub.1 and I.sub.2 will be able to be judged to be good but, on the contrary, in case I.sub.1 +I.sub.2 is smaller than I.sub.ref, the object will be at a far distance and the precision of the operating circuit 15 will be judged to be difficult to secure and, at this time, the switch 19 will be switched off and the switch 22 will be switched on so that the output I.sub.ref -(I.sub.1 +I.sub.2) of the subtracting circuit 23 will be input into the integrating circuit 20.
Now, the technical means suggested in the above-mentioned Japanese Patent Application Laid Open No. 291111/1989 has defects that, as two distance measuring means are switched over to one or the other by the value of I.sub.1 +I.sub.2, an erroneous switching may be made by a random noise and, as there is an unstable part near the switching of the distance measuring means, a discontinuity will be produced in the relation between the output value and distance.
Further, in the above-mentioned technical means, as such analogous divisional judgment as of I.sub.ref -(I.sub.1 +I.sub.2) is made in the far distance side operation, it has been difficult to measure the distance at a high precision on the far distance side.